When the Mars 2020 rover makes its journey to the Red Planet, the robot will have a special mission. It's going to pick up samples off the surface, put them in tubes and deposit them at various places on its journey. These little treasures will be left there for a few years until another mission can pick them up, retrieve them and bring them back to Earth, NASA said in a statement last month. "In laboratories on Earth, specimens from Mars could be analyzed for evidence of past life on Mars and possible health hazards for future human missions," the agency said.

Sample return has been a dream for scientists for many years. While rovers such as Curiosity are very capable science machines, the equipment they can bring to Mars is limited. Meanwhile, the U.S. is ramping up its capabilities in sample return generally with the OSIRIS-ReX mission, set to launch next month to fly to asteroid Bennu and bring back some bits of it for analysis on Earth.

What are the steps in Mars sample return? Here are some of the steps that NASA has outlined:

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With a whole planet to look at, how do you decide what to pick up? This actually begins in the planning process of the mission. Luckily for scientists today, we have numerous spacecraft orbiting Mars and taking high-resolution pictures from the surface. Usually NASA tries to land missions in areas that had water in the ancient past. The signature of hematite (which forms in water) is visible from space, and has been used to land missions such as Spirit, Opportunity and Curiosity. Once a rover is actually on the surface, selecting samples takes some time. High-resolution pictures are essential so that mission planners can figure out where they need to go, based on how similar the geology features on Mars look to the problem of interest as studied on Earth (say, looking for watery environments in the past). For example, when Curiosity found an ancient streambed shortly after its landing, one of its distinctive features was rounded rocks -- just like you see in streambeds on Earth. When the rover is up close, close-up photography is performed and if possible, other instruments are used to learn what kinds of elements are in the region. That's when mission planners can make the best choice about whether it's worth spending the time there to more drilling and analyzing of samples.

Image: The first rover on the Red Planet, called Sojourner, places a spectrometer over the surface of a rock nicknamed "Yogi" in 1997. Credit: NASA/JPL

Martian rocks and samples on the surface are interesting to scientists, but they live in a very undisturbed environment. Wind is the dominant feature of change on Mars and regularly you'll see dust storms, dirt devils and other disruptions that move things around. Mars also has a very thin carbon dioxide atmosphere that produces quite a bit of radiation on the surface, but so far as the rover Curiosity has seen, not so much to make a human mission impossible. The solution is to move deeper into Mars and extract samples from there. Both NASA and the European Space Agency will be practicing this skill with missions starting in 2018: both the InSight lander (NASA) and ExoMars rover (ESA) will carry drills to move deep into the Martian surface. The first drill ever to land on Mars is in place right now on the Curiosity rover. The first drill sample was taken in 2013, when it moved about 2.5 inches (6.4 centimeters) into the surface. On Curiosity, each sample is delivered to Curiosity's Collection and Handling for In-Situ Martian Rock Analysis (CHIMRA) device. The samples are closed in, shaken across a sieve to get the finest particles, and then moved to instruments inside. Curiosity has several spectrometers to analyze the elements inside the samples. Naturally, any samples slated to return to Earth would not be analyzed in this fashion, since scientists wouldn't want to contaminate the samples.

Image: Curiosity's drill prepares to move into the surface of Mars in this 2013 picture. Credit: NASA/JPL-Caltech

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When Mars 2020 looks to pick up samples, it will study rocks with two principal characteristics: rocks formed or altered by environments that could have supported microbial life and rocks that can preserve chemical traces of life. According to NASA, the rover will drill about 2 inches (5 centimeters) into the surface, break off a core sample, and seal a half-ounce (15-gram) sample inside each tube. The mission managers will then select where the rover should drop off each tube. "The sample cache(s) will remain on the Martian surface, awaiting potential pick-up by a future mission," NASA wrote. "Images taken by orbiters can identify locations of the samples with a precision of about 3 feet (~one meter). Images taken by the rover's own cameras will increase that precision to less than half of an inch (~one centimeter)."

Assuming that we can still find the samples and that they are still viable, a future rover would pick up the caches Mars 2020 left behind and bring them to an ascent vehicle. Bear in mind that we have never rocketed something off of Mars, so this would require a level of rocket science wizardry that still requires some research. The Apollo astronauts had a super-reliable rocket in the lunar module that minimized moving parts and mainly ignited by putting two volatile gases together. On Mars, we would have to find a reliable way to launch on a foreign world without a human on site to make changes.

Image: Artist's conception of a sample-return mission blasting off from Mars, within view of a rover (visible by its solar panel in the foreground). Credit: NASA/JPL-Caltech

Because Mars could have hosted biological organisms -- and Earth definitely does -- there will need to be special precautions when bringing the sample onto our planet. You can imagine that any capsule would have to be constructed strongly so that it does not break apart during re-entry, contaminating the sample and providing a small risk to Earth as well. NASA has a robust set of planetary protection constraints that are meant to minimize (ideally, eliminate) the microbes coming from Earth and for sample-return missions, to make the risk of contamination as small as possible. You can read some of the guidelines for robotic missions here.